This invention relates to improvements in the method of manufacturing measuring surfaces on measuring elements, particularly combination gauge blocks, surface plates, and the like.
Measuring surfaces such as are used on combination gauge blocks, surface plates, measuring tables, and measuring tips, must be extremely accurately formed if they are to provide their intended function. In many instances, it is required that the profile depth not exceed the order of 0.02 microns.
In the manufacturing process, measuring elements, such as steel combination gauge blocks, are susceptible to scratching, and thus there is a tendency that only some of the resulting measuring surfaces will have received sufficient smoothness while others are scratched so that they must be rejected or graded down to lower quality levels. By means of the present invention, however, it is possible to recover such rejected workpieces, at the same time imparting to them a much higher degree of scratch resistance. The method of the invention is applicable not only to the reworking of surfaces which have been rejected because of scratches, but it also is intended to be used as a preferred method in the making of measuring surfaces generally.
In many instances, such objects having measuring surfaces are made of hardened steel. To meet the demands regarding profile depth and exceedingly small dimensional tolerances, as well as to provide a high degree of flatness during the final working of said measuring surfaces, the procedure of lapping us used. Incidentally, "lapping" as used herein denotes an operation which is performed with abrasive between the workpiece and a rigid supporting means such as a cast iron disc, for example, and is to be distinguished from a polishing operation which is performed with an abrasive between the workpiece and a resilient supporting means such as felt or rasp. In general, a polishing operation is not satisfactory since the resiliency of the supporting surface is not conducive to obtaining the high degree of flatness required. In any event, in the lapping operation, an abrasive material which may comprise a finely grained powder of a hard, ceramic material such as aluminum oxide, silicon carbide, or diamond, is used. In the lapping operation, various conditions affect the smoothness of the final gauge surface such as the surface pressure, the lubricant, the difference in hardness between the workpiece and the working material, and unless all of these are at optimum conditions, scratching of the gauge surface may result, In some instances, as much as 60 percent of the measuring surfaces must be rejected or graded down to lower quality levels.
Attempts have been made to avoid some of the disadvantages referred to above by making the workpieces entirely of ceramic or cermet materials as, for example, hard carbides such as tungsten carbide or chromium carbide. The use of such materials has made it possible to avoid scratches; however, such materials have a significant disadvantage in that their coefficient of linear thermal expansion differs markedly from that of steel. Since measuring elements such as combination gauge blocks generally are used in the measurement of objects made of steel, and since it must be assumed that such measurements are carried out at different ambient temperatures, it is vital that the coefficient of linear thermal expansion of the material of the measuring element be substantially the same as that of steel.